The mole is the key concept for chemical calculations DEFINITION: The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12. Avogadro’s Number There are 6.022 x 1023 atoms in 12 grams of carbon-12. Therefore explained in simpler terms ‘One mole of any specified entity contains 6.022 x 1023 of that entity’: For most calculations we will do at AS we will use the following 3 equations moles = mass Mr 1. For pure solids and gases 2. For gases PV = nRT 3. For solutions Concentration = moles volume Learn these equations carefully and what units to use in them. Unit of Mass: grams Unit of moles : mol Unit of Pressure (P):Pa Unit of Volume (V): m3 Unit of Temp (T): K n= moles R = 8.31 Unit of concentration: mol dm-3 or M Unit of Volume: dm3 Note the different unit for volume Converting volumes cm3 dm3 ÷ 1000 cm3 m3 ÷ 1000 000 dm3 m3 ÷ 1000 Converting temperature oC K add 273 Typical mole calculations DEFINITION: Relative atomic mass is the average mass of one atom compared to one twelfth of the mass of one atom of carbon-12 DEFINITION: Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12 Some Simple calculations using above equations Example 2: What is the concentration of solution made by dissolving 5.00g of Na2CO3 in 250 cm3 water? moles = mass/Mr = 5.00 / (23 x2 + 12 +16 x3) = 0.0472 mol conc= moles/Volume = 0.0472 / 0.25 = 0.189 mol dm-3 Example 3: What is the mass of Cl2 gas that has a pressure of 100kPa, temperature 293K, volume 500cm3 . (R = 8.31) Example 1: What is the number of moles in 35.0g of CuSO4? moles = mass/Mr = 35.0/ (63.5 + 32.0 +16.0 x4) = 0.219 mol moles = PV/RT = 100 000 x 0.0005 / (8.31 x 293) = 0.0205 mol 100 kPa = 100 000 Pa 500 cm3 = 0.0005 m3 Mass = moles x Mr = 0.0205 x (35.5 x2) = 1.46 g Significant Figures Give your answers to the same number of significant figures as the number of significant figures for the data you given in a question. If you are given a mixture of different significant figures, use the smallest Remember the Mr must be calculated and quoted to 1dp chemrevise.org chemrevise.org 2 Avogadro’s Constant There are 6.02 x 1023 atoms in 12 grams of carbon-12. Therefore explained in simpler terms ‘One mole of any specified entity contains 6.02 x 1023 of that entity’: The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12. Avogadro’s Constant 1 mole of copper atoms will contain 6.02 x 1023 atoms 1 mole of carbon dioxide molecules will contain 6.02 x 1023 molecules 1 mole of sodium ions will contain 6.02 x 1023 ions Avogadro’s Constant can be used for atoms, molecules and ions No of particles = amount of substance (in mol) X Avogadro’s constant Example 4 : How many atoms of Tin are there in a 6.00 g sample of Tin metal? amount = mass/Ar = 6.00/ 118.7 = 0.05055 mol Number atoms = amount x 6.02 x 1023 = 0.05055 x 6.02 x 1023 = 3.04 x1022 Example 5 : How many chloride ions are there in a 25.0 cm3 of a solution of magnesium chloride of concentration 0.400 moldm-3 ? Number ions of Cl- = amount x 6.02 x 1023 = 0.0200 x 6.02 x 1023 = 1.20 x1022 (to 3 sig fig) amount= concentration x Volume = 0.400 x 0.025 = 0.0100 mol MgCl2 Amount of chloride ions = 0.0100 x2 = 0.0200 There are two moles of chloride ions for every one mole of MgCl2 Density density = mass Volume Density is usually given in g cm-3 Care needs to be taken if different units are used.
188.8.131.52 Relative atomic mass and relative molecular mass
Relative atomic mass and relative molecular mass in terms of 12C. The term relative formula mass will be used for ionic compounds. Students should be able to: • define relative atomic mass (Ar ) • define relative molecular mass (Mr ).
184.108.40.206 The mole and the Avogadro constant
The Avogadro constant as the number of particles in a mole. The mole as applied to electrons, atoms, molecules, ions, formulas and equations. The concentration of a substance in solution, measured in mol dm–3. Students should be able to carry out calculations: • using the Avogadro constant • using mass of substance, Mr , and amount in moles • using concentration, volume and amount of substance in a solution. Students will not be expected to recall the value of the Avogadro constant.
220.127.116.11 The ideal gas equation
The ideal gas equation pV = nRT with the variables in SI units. Students should be able to use the equation in calculations. Students will not be expected to recall the value of the gas constant, R.
18.104.22.168 Balanced equations and associated calculations
Students should be able to use balanced equations to calculate: • masses